This invention relates to variators.
In this context, a variator is a transmission component that interconnects two rotatable elements whereby, when rotating, the two elements have rotational speeds related to one another by a ratio (referred to as the “variator ratio”) that can vary between a minimum variator ratio and a maximum variator ratio in a substantially stepless manner. The invention may be embodied in a range of variator types, particularly those of the traction rolling type. Such variators may include ball bearing-and-ring type variators (such as Kopp variators), half toroidal variators, “Milner” variators and toroidal variators.
Full Toroidal Variators
The range of variators known as toroidal variators fall into two principal categories: “half-toroidal” and “full-toroidal” variators, so termed because of the toroidal (or half-toroidal) cavity that exists between input and output working surfaces of the variator. In a full-toroidal variator, each rotatable element drives a respective race within the variator, and the races rotate about a common axis (the “variator axis”). Each race has a working surface, arranged such that the working surfaces face one another in a direction parallel to the variator axis. An annular recess of arcuate cross-section is formed within each working surface, coaxial with the variator axis. The recesses are arranged such that they lie on a common hypothetical circle, the plane of which intersects the variator axis and the centre of which is in a plane (the “centre plane”) that is parallel to and spaced equally between the races. By extending the hypothetical circle around the variator axis, a hypothetical torus is described, the working surfaces occupying opposite regions of the boundary of the torus. Therefore, the space between the working surfaces of the races is referred to as the “toroidal cavity”. The radius of the centre of the hypothetical circle with respect to the variator axis is termed the “toroidal radius”, and the radius of the hypothetical circle is termed the “minor radius of the toroid”.
Typically, several rolling elements are provided within the toroidal cavity. Each rolling element is carried in a respective carriage. The rolling elements are free to rotate about a respective rolling element axis (normal to the plane of the roller) with respect to the carriage. Rotation of one of the races (called the “input race”) about the variator axis causes each rolling element that is in contact with it to rotate. This in turn causes the output race to rotate in the opposite direction to that of the input race. Similarly, application of a torque to the input race about the variator axis causes a torque in the opposite sense to be applied to the output race. During such rotation, the rolling elements will make contact with the input race and the output race about a respective circular contact locus described on the respective working surfaces. If these two loci are of the same radius with respect to the variator axis, then the output race and the input race will have the same rotational speeds. However, if the radius of the locus on the input race (the “input radius”) is not equal to the radius of the locus on the output race (the “output radius”), then the speed of the output race will be greater than or lesser than the speed of the input race. In general, the variator ratio (defined in this description as output speed divided by input speed) will be equal to the input radius divided by the output radius. Each carriage may be tilted such that the input and the output radii may be altered.
The variator just described may be termed a “single-roller” variator because there is only a single rolling element (albeit several may exist in parallel) in the power path between the races; typically each carriage therefore contains just a single rolling element. In the alternative case of a “twin-roller” variator, the rolling elements are arranged in pairs, and typically each carriage contains two rolling elements. Each rolling element has a race-contact rolling surface that makes contact with (subject to the discussion below) a respective working surface of one of the two races, and a roller-drive rolling surface that makes contact with the roller-drive rolling surface of the other rolling element of the pair. The race-contact rolling surface is part-spherical and the roller-drive rolling surface is frusto-conical. Rotation of one of the races (called the “input race” in this discussion) about the variator axis causes each rolling element that is in contact with it to rotate. This, in turn causes the other rolling element of the pair to rotate, which causes the output race to rotate in the same direction as the input race. Similarly, application of a torque to the input race about the variator axis causes a torque in the same sense to be applied to the output race.
Each carriage is configured such that it can be moved to alter the input and the output radii, this movement being referred to as “tilt”. At least when the variator is operating in an equilibrium condition, the input and the output radii are symmetrically disposed about the toroidal radius.
This specification refers to “contact” between the working surfaces and the rolling surfaces. However, this is a simplification adopted for convenience of description. Most embodiments of toroidal variators operate using traction drive. That is to say, the working surfaces and rolling elements are at least partially immersed or coated in a traction fluid. This has the property of having a viscosity that increases rapidly when its pressure exceeds a threshold. As the races rotate, traction fluid is drawn into the nips formed between the rolling elements and the working surfaces to create a thin layer of traction fluid between the rolling surfaces and the working surfaces, so there is, literally speaking, no direct contact between them. The term “contact” as used throughout should be understood to include contact through traction fluid as well as direct contact, as the context requires.
In order to achieve a satisfactory traction drive, an end-load is applied, which urges the races towards one another along the variator axis. The end-load is optimised to balance the requirement of providing sufficient loading to produce adequate traction at the interfaces between the working surfaces and the rolling surfaces, but low enough not to compromise the efficiency and durability of the variator. In many embodiments, the races may make slight movements along the variator axis in response to the end-load.
Within the general arrangement of a single-roller and twin-roller full-toroidal variator described above, a great many variations are possible concerning control, mounting and freedom-of-movement of the carriages, number and configuration of races, number and configuration of rolling elements, and so on.
The use of the terms “input” and “output” to define the races should not be taken as a functional or structural limitation relating to these components—they are simply labels. The variator may be entirely symmetrical in operation. These will typically be chosen to provide a concise and understandable description in a particular context. For example, in the case of transmission for a vehicle, the input will typically be connected to a prime mover, and the output will typically be connected to a final drive system to indicate the normal direction of power flowing through the variator. However, it will be understood that when the vehicle is in an overrun condition, engine braking will actually cause power to flow from the output to the input of the variator.
In the remainder of this specification, the term “variator” will refer to a single-roller full-toroidal variator as described above, unless the context indicates otherwise, but it will be understood that embodiments of the invention are not limited to such variators.